1. Field of the Invention
The invention concerns asynchronous digital data transmission systems. The invention is more particularly concerned with a method of reducing the low-frequency component of the jitter in a timing signal reconstituted at the output of a digital data transmission system of the type comprising at its input a higher order multiplexer receiving at least one stream of digitized data organized into blocks and having a particular timing, said multiplexer sending each block of data in a digital multiplex to the output of the transmission system via a transmission channel subject a variable multiplexing delay.
2. Description of the Prior Art
A digital data transmission system including a higher order digital multiplexing equipment is known to create jitter in the timing signal reconstituted at the output of the transmission system. A transmission system of this kind enables changes between different standardized or non-standardized hierarchical orders to apply to diverse transmission media a digital multiplex signal having specific characteristics (throughput, level, signal waveform, etc. The (in practise) asynchronous data streams arriving in parallel at the multiplexing equipment are received in storage means of the multiplexing equipment to allow for their asynchronous nature. The digital multiplex constructed from the stored data is transmitted at successive periodic times at the multiplexing equipment output timing. At the transmission system output the digital multiplex is decoded by means for decoding the multiplexed data such as a demultiplexer, the decoding means reconstituting a timing signal used to extract the data from the multiplex on the basis of the times at which data appears. The reconstituted timing signal at the transmission system output is affected by jitter which is due to fluctuations in the multiplexing delay conditioned by phase relationships between the times of arrival of the data at the multiplexer input and the opportunities to send said data in the multiplex. The jitter affecting the reconstituted signal is more accentuated when multiplexing is carried out on blocks of data (packet multiplexing) rather than on binary units (bit by bit multiplexing). Taking the example of a packet multiplexer in which the packet input timing is close to a submultiple of the multiplexed packet output timing, the relative phase of the packet arrival times and the packet sending times is subject to long term slip because of the unfavorable timing ratio which produces one or more phase jumps in the reconstituted timing signal. This low-frequency component of the jitter causes errors in decoding the multiplex data such as incorrect routing of the demultiplexed data.
A known way to reduce the jitter is to choose a ratio between the relative timings of the input tributaries of a higher order multiplexer and the multiplex signal at the multiplexer output which is sufficiently complex to achieve high-frequency phase beating that is likely to be easier to filter. CCITT Recommendation G.742, for example, describes a method for multiplexing four 2 048 kbit/s channels into an 8 448 kbit/s multiplex. This multiplex uses a frame of 848 bits repeated approximately every 100.38 microseconds. Such ratios complicate the transmission system, however, imposing operating constraints and limiting its flexibility of use.
Reducing the jitter has also been attempted by increasing the effectiveness of the reconstituted timing signal filter using a phase-locked loop. These techniques increase the time to synchronize the transmission systems, however, because the synchronization time is conditioned by the time constant of the loop.
Procedures for using signals which tolerate a high level of jitter are known and complement the jitter reduction techniques in the digital data transmission systems previously mentioned. They are extremely complex and somewhat unreliable, however. Also, the low-frequency component of the jitter is still present. Consequently, an object of the present invention is to alleviate the drawbacks of the techniques described above.
Differing in this respect from the known techniques described above, the invention starts from the observation that it is preferable to attempt to reduce the jitter in the reconstituted timing signal by controlling the process which generates the jitter, in other words the higher order multiplexer. This is achieved by forcing the multiplexer to introduce an appropriate multiplexing delay so that the multiplexing delay variations compensate each other and the mean multiplexing delay is constant in the long term. This eliminates the low-frequency component of the jitter in the reconstituted signal at the transmission system output. This method is simple and inexpensive to implement.